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Author Topic: impedence mismatch question  (Read 731 times)

Offline IQ_imbalance

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impedence mismatch question
« on: September 02, 2018, 1559 UTC »
This is based off an observation i made trying to hook my H-800 directly into my afedri while on a road trip.  The receiver's HF input is 50 ohm per the specs, and the H-800 uses RG-174 (50 ohm) so I didn't anticipate any problems.  HOWEVER, the power supply box for the H-800 (the antenna probe receives power over coax from the box, and the connection to the receiver is on the box) uses RCA-type connectors for some reason while the afedri has those tiny SMA connectors.  No problem i thought, since i have a spectrum of various adapters.  So i hook everything up, with an RCA to F-type coupled to a F-type to SMA which is plugged into the afedri (I did NOT have a RCA to sma hence the workaround).  Long story short..the receiver was deaf as a bat.  Thinking there might be a problem w/ the H-800, i decided to rig up an improvised LOG using the RG-174 as it was the only coax i had.  Still nothing...which was very odd considering i've had very good luck with LOG from that remote location before.  The coax was visibly intact and had good conductivity between the ends.  I finally considered the option that there might be an issue w/ the adapters and did an improvised connection (e.g. cut the RCA off the coax and directly made conductivity between the coax and the SMA connector on the radio) and BOOM!  everything working great. 

So...the adapter seems electrically fine in terms of conductivity.  My working theory was that there was a problem with the 50 ohm-75 ohm-50 ohm mismatch between the coax, the adapters, and the radio.  Does that make any sense at all?  If so...why do i get OK results going from an antenna through 75 ohm RG-6 and into the radio with an F-type to SMA connector?

LOG/NE-SW unterminated BOG
AFEDRI SDR-Net
Central MD

Offline ChrisSmolinski

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Re: impedence mismatch question
« Reply #1 on: September 02, 2018, 1630 UTC »
A 50 vs 75 ohm mismatch won't cause the receiver to be deaf. That kind of a mismatch is trivial for an HF radio  ;D

I suspect either a faulty RF adapter/connector, or possibly your first attempt did not include necessary DC blocking that your second did, so the receiver shorted the antenna's power. I don't recall offhand if the AFEDRI has an A/C coupled input, I would think it does, but maybe not. Or perhaps something in your first attempt had a DC short to ground?
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Offline pinto vortando

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Re: impedence mismatch question
« Reply #2 on: September 02, 2018, 1923 UTC »
A 50 vs 75 ohm mismatch won't cause the receiver to be deaf.


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Offline ThElectriCat

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Re: impedence mismatch question
« Reply #3 on: September 09, 2018, 1530 UTC »
Chris Smolinski and Pinto Vortando are right about the 50/75 mismatch not causing any trouble on RX, but if you are using a powered antenna, make sure whatever adapters you are using are allowing it to get the right voltage, and enough current. otherwise it will be effectively an open circuit. ie, deaf
In another life, I could have been a telephone engineer.

Offline Josh

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Re: impedence mismatch question
« Reply #4 on: September 09, 2018, 1900 UTC »
Chris Smolinski and Pinto Vortando are right about the 50/75 mismatch not causing any trouble on RX, but if you are using a powered antenna, make sure whatever adapters you are using are allowing it to get the right voltage, and enough current. otherwise it will be effectively an open circuit. ie, deaf

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